PROJECT SUMMARY/ABSTRACT According to the most recent CDC report, 1 in 68 children are diagnosed with an autism spectrum disorder (ASD). With 30?40% of individuals with ASDs developing epilepsy, epilepsy represents a major source of morbidity and mortality associated with these disorders. The cellular and molecular mechanisms responsible for epilepsy in individuals with ASDs remain largely unknown. We have developed a new method for making human cortical organoids with functional neural networks from induced pluripotent stem cells (iPSC) and generated iPSC from Phelan-McDermid syndrome (PMDS) patients with epilepsy. Loss of SHANK3 is considered to be the main cause of the neurological and psychiatric deficits in PMDS patients. The main goal of this project is to determine whether and how loss of SHANK3 causes the development of overexcitability deficits in human cortical networks using cortical organoids generated from control and SHANK3-deficient iPSCs. The central hypothesis of this project is that reduced SHANK3 expression in cortical excitatory neurons leads to development of increased intrinsic excitability and seizure-like activity in cortical networks. The following Specific Aims will be completed to test this hypothesis: 1) we will determine the identities of neurons with increased intrinsic excitability in SHANK3-deficient cortical organoids and 2) we will determine whether SHANK3-deficient organoids develop elevated electrical activities in neural networks. The proposed research is significant because it is expected to substantially advance understanding of the cellular and molecular deficits associated with epilepsy in patients with neurodevelopmental disorders and to provide a new platform for epilepsy research and drug discovery.